Roger Federer started his bid for a seventh ATP Finals title with a win but Jack Sock showed he will be a star of the future.

Across London from where he won his eighth Wimbledon title in July, the Swiss superstar needed 91 minutes under The O2 roof to beat the American No.1 6-4 7-6.

But Sock, who only qualified for the event for the first time by winning the Paris Masters last week, fought hard in the entertaining second set to force a tie-break before Federer clinched victory.

The Swiss, who is making his 15th appearance at the season-ending event after missing last year with injury, said: “It is great to be back here.

Sock showed he is a star of the future (Image: Getty Images Europe)

The two shake hands after the game (Image: Getty Images Europe)

"Today was really just a matter of trying to keep the ball in play. The first round in any tournament is always a struggle because you are not used to the conditions and you are not sure how the ball will fly.

"I got off to a great start and the second set was tight. The breaker could have gone either way. He helped me with some double faults and errors.”

The two stars wore matching grey Nike kits but Federer was clearly the standout player in the 37-minute first set.

Sock's only respite came when he played a poor half-volley at the net and turned his back to his opponent only for Federer to push his forehand into the net with the court wide open.

Federer is bidding for a seventh title (Image: AFP/Getty)

The American, 25, saved five break points on his serve in the second set but was unable to break the Federer serve all match. He committed a crucial double fault at 4-4 in the tiebreak to allow Federer to serve out the match.

World No.2 Federer, who last won here in 2011, is the favourite to win here with Andy Murray and Novak Djokovic out injured while world No.1 Rafa Nadal is not a fan of playing on indoor hardcourts and has never won this title.

The Spaniard was presented with the trophy for finishing the year at the top of the rankings after the match.

World No.3 Alexander Zverev will face Marin Cilic in the second singles match tonight. Jamie Murray and his partner Bruno Soares begin their doubles campaign tomorrow.